1. Field of the Invention
The present invention relates to ceramic powders and sintered materials made from such powders. Such materials find particular utility as metal forming members such as metal cutting and forming tools.
2. Background of the Invention
During the mid-1930""s, tool steel alloys began to be replaced by sintered tungsten carbide powder tools, which quickly became the standard because of their excellent hardness and inherent high toughness and transverse mechanical strength. The hardness of such materials improved tool life, and the toughness and strength helped increase productivity by allowing higher feeds, speeds, and more aggressive forging parameters. Carbide tool development and commercial availability increased significantly after World War II.
Even these materials eventually wear, and the mechanisms of such wear are not as yet fully understood. Progressive wear causes variation in the materials being shaped, and as a result of the need to hold part dimensional tolerances, the tool must be replaced when it is no longer able to form the part to the correct dimension. The time or number of parts formed before such an occurrence ultimately determines the limit of the tool""s life. The resulting productivity loss during tool change out and process readjustment, nonconforming production, rework, and missed schedules have been a driving force for obtaining materials that provide longer tool life.
Tool life is determined by its resistance to several types of wear, its response to heavy loads, and to shock. In general, the higher the chip removal rate (high feeds and speeds), drawing and forming pressures, and the longer tool geometry is retained, the better the tool. Superior cutting and forming tools must be simultaneously hard, strong, stiff, and resistant to chipping, fracture, heat breakdown, fatigue, chemical reaction with the work piece, and attrition wear. Accordingly, the dominant desirable mechanical properties sought in a sintered tool are strength, hardness, high elastic modulus, fracture toughness, low chemical interaction with the work piece, and low coefficient of friction to aid work piece forming while reducing heat buildup.
In recent years, the powder metallurgy (PM) industry has increased significantly because of the ability of powders to flow cold into a precision mold. This allows the mold to be reused, often at high volume, while dramatically reducing machining, forming, and other process steps because the sintered part is already very close to its intended configuration, or xe2x80x9cnear net shape.xe2x80x9d Increasingly these parts, now produced principally of aluminum, ferrous, and copper powders, require some of the same desirable attributes as tools. For this reason, many PM articles undergo additional forging, plating, or heat treatment operations to develop localized hardness, toughness, and strength. Many of these parts require resistance to shock and abrasion that call upon the same mechanical properties as are required for tools.
In tools and hard articles, wear resistance is increased at the expense of strength; today, the best tools exhibit the best compromises, and therefore are limited for use in special applications.
Beyond tungsten carbide, various alloys, coating techniques, and combinations of both have been found to permit not only longer tool life but also increased cutting speeds and feeds. Powder metallurgy and sintering have lead to the development of new materials with enhanced hardness and toughness, and adding a hard coating to the sintered alloy such as by chemical vapor deposition (CVD), physical vapor deposition (PVD), or plasma-assisted chemical vapor deposition (PACVD) has increased wear resistance.
Much is taught in the prior art about preparation of coatings on powders, coating substrates, and other hard material enhancements. The prior art of tool materials teaches six approaches that are currently known and in general use for the achievement of such enhanced wear resistance and toughness; each having significant benefits and significant drawbacks: (1) mixing hard and tough phase particles, (2) chemical vapor deposition (or other) coating of sintered substrates with hard phase layers, (3) combining approaches one and two, (4) cermetallic (cermet) compacts, (5) for a special type of tool (grinding and sanding media), chemically bonding low concentrations of large diamond or cBN particles into a hard but relatively weak abrasive substrate, and (6) Functionally Gradient Materials (FGM).
None of these solutions has brought about the essential combination of desired tool properties, and only the chemical vapor deposition (CVD or PVD) approach is today applicable for some mechanical parts requiring increased abrasion resistance.
Mixing Hard and Tough Ternary Systems.
In spite of the many ancillary treatments and variations that exist and that are taught in the art, mixing hard WC-TiN-Co alloy particles with the carbide powder before sintering has several disadvantages. Because these harder particles have low mutual solubility with the binder, substrate transverse strength drops quickly above 6-10 wt. percent hard particles. Surface hardness and wear resistance are accordingly reduced also, compared with a surface coating. The wear mechanism is also not greatly enhanced because the few hard particles (less than one in ten at the surface where needed), weakly bound to the binder, break away whole.
Chemical Vapor Deposited (CVD) Coatings.
These hard external coatings of hard intermetallic and cermet layers on tool steels or sintered article substrates (after sintering) are valued for the high surface hardness they impart, typically exhibiting values of 2400 Vickers (TiN) to 5000 Vickers (cubic boron nitride) to 9000 (diamond). Yet, for all the ancillary treatments, variations, and sintering aids that exist and are disclosed in the art, including additional coating layers, locally altered substrate structures, and grain-size reducing dopants or coatings, the external coating solution has several major disadvantages, including coating delamination and cracking in use (from different coating and substrate thermal expansion rates and from bending and surface loads) and the high CVD process temperatures required (900xc2x0 C.-1200xc2x0 C.) may not be consistent with the heat-treatment needed for the strength or the geometry of the sintered part.
Conventional CVD coating of already-sintered articles with several different coatings or layers allows them to resist two or three unique work piece challenges. But since each layer must be deposited sequentially the remaining one or two special coatings must remain covered until the outer layers have worn away. Therefore, only one of the concurrent substrate coating design challenges can be met at the same time.
Some categories of tools, such as drawing dies and nozzles, are even more prohibitively expensive because there is an additional cost to assure the CVD vapor is adequately circulated through the die orifice for the deposition of a coating, where it is most needed. Diffusion of the CVD gas is slow and penetration is typically 0.5 to 10 micrometers or less. First, at these thicknesses, the coating is worn through to the underlying carbide before most of the wire or tube diameter tolerance is used up. Second, the normal reutilization of the dies at larger diameters must be done without the hard coating. In many cases, tool total life prolongation may not be proportional to the added CVD cost.
Today, external coatings are the most common commercial solution to enhancing the performance of plain sintered tungsten carbide products. Increasing the deposition thickness of outer layers to gain greater life has diminishing returns; it tends to increase the propensity for cracking and to round off sharp tool edges, adversely affecting optimal cutting or die geometry.
Combined Mixes and Coatings.
CVD coating and mixing hard alloy particles, a combination of (1) and (2) above gives very limited added benefit while having the same drawbacks.
Cermets.
Cermets are ceramic particles dispersed in a metal oxide or carbide matrix. Cermets combine the high-temperature resistance of ceramics with the toughness and ductility of carbides. They are priced about the same as plain tungsten carbide, and wear is about the same, except for light finishing cuts, where it outperforms plain carbide.
Sintered Abrasive Composites.
The fourth approach, taught in Dr. Randall M. German""s book, Liquid Phase Sintering, Plenum Press, New York 1985 (and practiced in Russia many years earlier) creates a class of super abrasive composites for grinding and sanding media and niche application tools.
Such composites are produced by mixing diamond particles (or cubic boron nitride, cBN) and cobalt powders or capturing them in a metal (nickel) electroplate deposit and hot pressing these at lower temperatures. An alternative is to coat the diamond (or cBN) with an intermediate layer of a transition metal carbide former (which wets the diamond) and chemically bonds it to others with a low melting point nonwetting but ductile metal binder such as cobalt, iron, or nickel. The transition metal is applied solely as a chemical bridge in thicknesses not intended to bear structural mechanical load. The metals used as the principal binder matrix have good sinterability but relatively low melting points, elastic moduli, and strengths. Such materials have desirable properties in abrasive applications. In most of these applications the diamond represents 10 to 60 vol. percent of the composite. The binder coatings are several micrometers thick, to aid processing at low temperatures (to avoid graphitic degradation of the diamond) and dilute the diamond content, but with great sacrifice in mechanical properties. The properties of these composites are governed by chemical considerations, not mechanical considerations of elastic modulus, strength, or fracture toughness. Accordingly, with the large diamond particle size and large binder concentration, the mechanical properties of the composite are determined by the rule of mixtures. The compositions are selected to ensure the diamond particles are separated in the final microstructure, assuring there can be little diamond-diamond interaction. There is little amplification of mechanical advantage as is found in the one micrometer to nanoscale grain size range of sintered carbides.
The requirements for grinding tools are relatively large grains (50 to 600 micrometers) to increase metal removal, bonding these particles to a wheel, adequate spacing between particles (low concentration of particles with large binder phase ligaments) to permit removal of work piece particles, and long retention of grinding wheel geometry. Such materials form metals by wearing away the work piece by the sheer differential in hardness between the abrasive particles and the work piece itself. Such abrasive composites are sometimes used in cutting tools used in machining specialty high hardness materials at relatively high speed but very low chip removal (load) rates. (See FIG. 6). The cutting action of diamond cutting tools is very different from that of cemented carbide tools. The limitation of diamonds or composites in cutting tools stems from their cutting behavior. Such composites operate as abrasives, where they generally perform by wearing away the work piece, rather than by removing chips under heavy load. In this mode, the very hard diamond particle is retained by a tensile bond. While sliding across the work piece, the diamond becomes exposed to cut the opposing surface, but it resists wear while the matrix erodes and progressively exposes the diamond. It is the protruding diamond that performs cutting as long as its remains sharp. When the diamond dulls, it becomes rounded and the matrix is designed to fail. In this manner the diamond is pulled out by the work piece and the matrix erodes until another diamond is exposed.
Such hard, brittle abrasive composites are also used in some tool-like applications such as masonry bits and saws. They are also found in high cost wire drawing dies, and some cutting tools where their performance is permitted by presence of steel or other strong backing.
Functionally Gradient Materials (FGM).
The problem with coated articles is the incompatibility between mechanical, chemical, or thermal properties of the layers. To correct this problem by providing a gradual transition between incompatible layers, FGM""s have one or more of the following variables: chemical composition, microstructure, density, or variable forms of the same material. Another purpose is as a coating to modify the electrical, thermal, chemical or optical properties of the substrate upon which the FGM is applied. The principal disadvantages of such materials is their tendency to fail at locations where the properties change and the difficulty in manufacturing such materials.
The principal objective of the present invention is to create sinterable particulate materials called Tough Coated Hard Powders (TCHPs) that provide increased value over hard article and tool materials known today. The particles and articles made therefrom combine the best mechanical properties of strength, hardness, high elastic modulus, fracture toughness, low interaction with the work piece, and low coefficient of friction that exist separately in conventional materials into an article of unmatched properties.
It is a further object of the invention to reduce the cost of providing such materials to users. For example, tool inserts must be provided in considerable geometric variation to fit into the various toolholders. Moreover, the tool materials available today must be designed for very specific applications. Therefore, for each of these geometric variants, material choices (uncoated, CVD coated, PVD coated, cermet, ceramic, polycrystalline cBN, polycrystalline diamond) must also be offered. The combination of geometric and material variations requires expensive catalogs, needless tool manufacturing redundancy, costly supplier and user inventories with unique packaging and identification, and sales effort to explain and sell the confusing array to users. Another object of the present invention is to reduce the waste and cost associated with the present system by providing more general purpose, higher-performance tools at reasonable cost.
In addition, the process of making the product embodiments of the invention has the object of reducing the cost of production of articles made in accordance with the invention.
Another object is to provide significant cost reduction by extending the first-time article life and by decreased manufacturing cost of the products they touch. The fact that the articles of the present invention are macroscopically homogeneous, rather than coated, offers users or suppliers the opportunity of economically regrinding and reusing the initially worn articles.
Yet another object of the invention is to provide the same high performance mechanical properties of the materials of the present invention to other hard article applications.
Another objective of the present invention is to provide a material having enhanced wear resistance and toughness for use in a broad array of articles including tooling (such as drawing dies, extrusion dies, forging dies, cutting and stamping dies, forms, forming rollers, injection molds, shears, drills, milling and lathe cutters, saws, hobs, broaches, reamers, taps and dies); individual mechanical parts (such as gears, cams, journals, nozzles, seals, valve seats, pump impellers, capstans, sheaves, bearing and wear surfaces); integrated co-sintered components to replace mating parts (internal combustion engine connecting rods, bearings) and/or to provide hard surface zones in powdered metal (P/M) mechanical parts substituted for forged or machined steel parts with heat treated zones (such as camshafts, transmission parts, printer/copier parts); heavy industrial articles (such as deep well drilling bits, teeth for mining and earthmoving equipment, hot rolls for steel mills); and electromechanical components (such as memory drive reading heads, specialized magnets). In addition to providing such novel articles, principal objectives of the invention are to provide novel composite particulate materials (i.e., TCHP""s), novel methods for producing such materials, and novel methods for fabricating articles from such materials.
To achieve these and other objects there is provided a sintered material comprising a plurality of core particles that consist essentially of a first metal compound having the formula MaXb. M is a metal selected from the group consisting of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, aluminum and silicon. X represents one or more elements selected from the group consisting of nitrogen, carbon, boron and oxygen and a and b are numbers greater than zero up to and including four. An intermediate layer surrounds each of the core particles and consists essentially of a second metal compound, different in composition from said first metal compound. The second metal compound has a higher relative fracture toughness and is capable of bonding with the first metal compound and also is capable of bonding with iron, cobalt or nickel. The core particle with the intermediate layer thereon forms a number of coated particles. An outer layer overlays the intermediate layer on the coated particles and functions as a binder. It is comprised of iron, cobalt, nickel, their mixtures, their alloys or their intermetallic compounds.
Preferably the coated particles have an average particle size less than about 2 xcexcm and most preferably less than about 1 xcexcm. It is also preferred that the intermediate layer have a thickness, after sintering, in the range of from 5% to 25% of the diameter of the core particles. It is also preferred that the outer layer have a thickness after sintering in the range of from 3% to 12% of the diameter of the coated particles. It is believed that with such a thickness of the outer layer that strain fields associated with dislocations in one coated particle are transmitted through the outer binder layer to the immediately adjacent intermediate layer. Preferably the first metal compound consists essentially of a stoichiometric compound such as TiN, TiCN, TiB2 TiC, ZrC, ZrN, VC, VN, cBN, Al2O3, Si3N4 or AlN. It is also preferred that the second metal compound consist essentially of WC or W2C, and most preferably WC. Such materials have a fracture toughness greater than cubic boron nitride.
A preferred embodiment of a sintered material comprises a plurality of core particles consisting essentially of cubic boron nitride with an intermediate layer on each of said core particles said layer consisting essentially of WC. The intermediate layer has a thickness, after sintering, in the range of from 5% to 25% of the diameter of the core particles. An outer layer comprising cobalt or nickel overlays the intermediate layer and that outer layer has a thickness after sintering in the range of from 3% to 12% of the diameter of the coated particles. The combination of the core particles, intermediate layer and outer layer forming a coated particle, preferably having an average particle size less than about 1 xcexcm.
Another embodiment of the present invention is a powder consisting essentially of a plurality of coated particles. The majority of the coated particles have core particles consisting essentially of a first metal compound having the formula MaXb. M is a metal selected from the group consisting of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, aluminum and silicon. X represents one or more elements selected from the group consisting of nitrogen, carbon, boron and oxygen and a and b are numbers greater than zero up to and including four. The core particles coated with a surrounding layer consisting essentially of a second metal compound, different in composition from said first metal compound and having a higher relative fracture toughness. The layer also is capable of bonding with the first metal compound and also capable of bonding with a metal selected from the group consisting of iron, cobalt and nickel. Preferably the coated particles have an average particle size of less than about 2 xcexcm and most preferably less than about 1 xcexcm. It is also preferred that the layer surrounding the core particles after sintering have a thickness in the range of from 3% to 200% of the diameter of the core particles.
The preferred compositions of the core particles and surrounding layer (the intermediate layer) are the same for the powder embodiment as for the sintered article.
It is also preferred that the outer binder layer consist essentially of cobalt, nickel, iron, their mixtures, their alloys or their intermetallic compounds deposited on the outer surface of the second metal compound layer in the form of a continuous layer.